The thyroid hormones, triiodothyronine (T₃) and its prohormone, thyroxine (T₄), are tyrosine-based hormones produced by the thyroid gland that are primarily responsible for regulation of metabolism. T₃ and T₄ are partially composed of iodine (see molecular model). A deficiency of iodine leads to decreased production of T₃ and T₄, enlarges the thyroid tissue and will cause the disease known as simple goitre. The major form of thyroid hormone in the blood is thyroxine (T₄), which has a longer half-life than T₃. In humans, the ratio of T₄ to T₃ released into the blood is roughly 20 to 1. T₄ is converted to the active T₃ (three to four times more potent than T₄) within cells by deiodinases (5'-iodinase). These are further processed by decarboxylation and deiodination to produce iodothyronamine (T₁a) and thyronamine (T₀a). All three isoforms of the deiodinases are selenium-containing enzymes, thus dietary selenium is essential for T₃ production.